#include <bits/stdc++.h>

using namespace std;

namespace {

    typedef double real;
    typedef long long ll;

    template<class T> ostream& operator<<(ostream& os, const vector<T>& vs) {
        if (vs.empty()) return os << "[]";
        os << "[" << vs[0];
        for (int i = 1; i < vs.size(); i++) os << " " << vs[i];
        return os << "]";
    }
    template<class T> istream& operator>>(istream& is, vector<T>& vs) {
        for (auto it = vs.begin(); it != vs.end(); it++) is >> *it;
        return is;
    }

    string A, B;
    void input() {
        cin >> A >> B;
    }

    bool C() {
        if (A.size() < B.size()) {
            return true;
        } else if (A.size() > B.size()) {
            return false;
        } else {
            int N = A.size();
            assert(N == B.size());
            for (int i = 0; i < N; i++) {
                if (A[i] == B[i]) continue;
                if (A[i] == '4' && B[i] == '7') {
                    return false;
                }
                if (A[i] == '7' && B[i] == '4') {
                    return true;
                }
                return A[i] < B[i];
            }
            assert(false);
        }
    }

    void solve() {
        cout << (C() ? B : A) << endl;
    }
}

int main() {
    input(); solve();
    return 0;
}